Etalon-lens-fiber (ELF) optical assemblies have many practical applications. One is to impose a group delay on the wavelength components of light to correct group velocity dispersion (GVD) previously induced on the light's pulses by a high speed, long haul, Dense Wave Division Multiplexing (DWDM) transmission system. For example, an etalon typically has a first mirror that is partially reflective, a second mirror that is fully reflective and a glass cavity in between. The spacing between the mirrors (i.e. the thickness of the glass cavity) is generally a function of the channel spacing of a DWDM system in which the optical assembly is operative. Light arriving from a lens enters and exits the etalon through the partially reflective mirror. The etalon subjects different wavelength components, i.e. different frequencies, of the light to variable delay. That is, the partial reflectivity of the first mirror causes certain wavelength components to be restrained in the glass cavity between the first mirror and the second mirror longer than others, with the wavelength components restrained the longest said to be at resonant frequencies. The etalon thereby imposes a group delay on the wavelength components of the light which can correct group velocity dispersion previously induced on the light's pulses by a high speed, long haul, DWDM transmission system.
One technical challenge presented by using ELF and similar optical assemblies in practical applications is how to address the frequency dependence of the insertion loss and insertion loss ripple of such assemblies. Because different wavelength components of light incident to etalons bounce between the front and back mirrors a different number of times prior to transmittance, light reflected from etalons exhibits a frequency-dependent spatial shift and a phase curvature. As a result of this shift and curvature, certain frequencies of light outbound from the etalon transmit on the outbound fiber more efficiently than others. This difference in transmission efficiency among frequencies is evident in frequency-dependent insertion loss and insertion loss ripple profiles.
Previous solutions have attempted to control the frequency dependence of the insertion loss and insertion loss ripple inherent in ELF and similar optical assemblies by introducing spectral filters into the system. However, there are disadvantages to this approach. First, spectral filters increase the insertion loss of the system by the average loss of the spectral filter. Second, spectral filters have typically only been able to make modifications to insertion loss that are slowly varying with frequency.